Reader:

TheWaterBudget

LearningOutcomes:Thepurposeofthislabistobeabletoidentifyand key soil-water budget components and to be able to calculate and plot thesoil-water budget for a selected location. The lessons learned from this laboratorycould,inthefuture,byappliedbythestudenttowaterresource analyses of different hydro-systems including watersheds, reservoirs and regions as open systems with inputs, outputs and storages of water.

Materials:Pencil,coloredpencils,labhandout,simplecalculator.

PartI.Whatisthewaterbudget?

Waterisnot always naturally available when and where itisneeded. Hence,humansmustrearrangewaterresources.Themaintenanceofahouseplant,thedistribution of local water supplies, and irrigation program on a farm, the rearrangement of river flows — all involve aspects of the water balance and water-resourcemanagement.

Thewaterbudgetisanaccountingoftheincomingandoutgoingcomponentsofwaterfromacontrolvolume(e.g.awatershed,asoilcolumn,afarm,alake,etc).SuchabudgetcanbeestablishedforanygivenareaontheEarth’ssurface

—acontinent,nation,region,orfield.Forasoil-waterbudget,itisimportantto measurealltheprecipitationinputanditsdistributiontosatisfythe“demands” ofplants(transpiration),evaporation,runoff,soilmoisture,groundwater,waterbodiesorsnow(ice)storageintheareaconsidered.Suchabudgetcanexamineany time frame, from minutes to years.

Whenthereisnotsnow/iceorpondedwaterstorage,thentheonlywaterstorageoccurs into the soil. So, think of a soil-water budget as a money budget: precipitationincomemustbebalancedagainstexpendituresofevaporation, transpiration, and runoff. Soil-moisture storage acts as a savings account, accepting deposits and withdrawals of water. Sometimes all expenditure demands are met, and any extra water results in a surplus. At other times,precipitationandsoilmoistureincomeareinadequatetomeetdemands,andadeficit,orwatershortage,results.Thewaterbalancedescribeshowthewater supplyisexpended.Thinkofprecipitationas“income”andevapotranspiration